    [Patent document 1] JP-2005-337229 A (US-2007/02139920 A1)
Generally, under the state where the speed of a vehicle comes to be some extent, the motion energy of the vehicle enables an inertial travel of the vehicle, without need of an accelerator being stepped on. This can typically reduce the fuel consumption.
Patent document 1 discloses a technology as follows. The above inertial travel is detected from the state where the accelerator opening degree is zero. The travel distance or travel duration of the vehicle under the inertial travel is then obtained. The ratio of the obtained travel distance or travel duration under the inertial travel is calculated with respect to the total travel distance or the total travel duration. The calculated ratio and the relevant advice are notified. Assistance or management is thus provided to the driver for the fuel saving driving operation.
Incidentally, the travel speed of the vehicle may needlessly change by repeated useless operations of the accelerator during the travel. In such cases, the speed accelerated by the useless operations of the accelerator eventually needs to be slowed down by the inertial travel. Although the travel distance under the inertial travel becomes long compared with that of the usual inertial travel, the energy loss arises at the time of the acceleration and deceleration. This poses a disadvantage to increase the amount of fuel consumption accordingly.
As explained above, the technology in Patent document 1 detects the inertial travel from the state in which the accelerator opening degree is zero; thus, it cannot provide an appropriate diagnosis for the fuel saving driving operation in the state where the travel speed of the vehicle changes more than needed during such an inertial travel.